Various industrial applications require the selective picking of sheets from a stack to feed a machine or process, such as sheets of paper for printing presses, sheets of metal for punch presses, plastic sheets for laminating machines, and the like. The process of retrieving sheets from a stack is commonly referred to as stack feeding. When feeding flat flexible substrates form a stack, one issue that commonly arises is adhesion between the individual sheets, and especially adhesion of the topmost sheet with one or more sheets that are below it in the stack. Such adhesion, which may impair the separation of the uppermost sheet from a second or even several additional sheets, as the topmost sheet is picked from the top of the stack for insertion into a machine or process, is an undesired issue that is commonly referred to as multi-picking.
The adhesion between sheets that can create issues for stack feeding applications is typically the result of one or more factors, such as the weight of the sheet substrates, friction between the sheets, static electrical charges of the sheets, which is an especially predominant factor for glossy and coated sheets, and other factors.
Various solutions have been proposed in the past to aid sheet separation in stack feeding applications. In one previously proposed solution, ventilation of the top few sheets of the stack is used to separate the sheets by a thin layer of air. In this solution, the sheets are ventilated by injection of an air stream between sheets lying on the top of the stack. The injection of air may be accomplished by a fan or a compressor that is configured to provide an air stream onto the desired area of the stack. Alternatively radial or axial fans have been used in the past to provide the air stream, which is then delivered to the top of the stack by use of ducts and guides.
Although known ventilation arrangements are at least partially effective in providing separation between sheets, these known devices are heavy, expensive, and not particularly capable in providing consistent sheet separation when the stack height is being reduced as sheets are removed. Moreover, the ducts and other devices used to direct and deliver the air stream cause losses in the air delivery system for the stream and are not easily adaptable to various stack heights or for larger sheets.
One example of a known sheet separation arrangement for a stack feeding process can be seen in U.S. Pat. No. 6,729,614 (the '614 patent), which is entitled “Sheet Feeding Apparatus.” The '614 patent describes (see, for example, FIGS. 1(a) and 1(b)) a roller based feeding system. The roller based feeding system includes side blowers having a fixed position relative to a stack for providing a stream of air between sheets that separates the sheets. The side blowers include radial fans and diverters to route the stream of air towards the stack. In the illustrated embodiment, the stream of air is bent by over 90° via nozzles, which direct the stream towards the side walls of the sheets. Adjustments to the position of the topmost sheet in the stack relative to the blowers as sheets are removed from the stack is accomplished by a lift table, which is configured to raise the stack as sheets are removed from the top of the stack.
An additional known example of a sheet separation arrangement can be seen in U.S. Pat. No. 6,015,144 (the '144 patent), which is entitled “Sheet Feeder and Image Forming Apparatus.” In one embodiment, the '144 patent describes a roller based sheet feeder that uses side blowing means providing an air stream through a side wall of a tray containing the stack (see, for example, FIGS. 28A-28C). The air is supplied by a blower or fan that is fixed to the side wall of the tray. The devices disclosed in the '144 patent further include air regulation members that can direct the air stream to a desired location in the stack from the fixed-position blower or fan. Moreover, insofar as the position of the blower is fixed, either the air regulation members or the height of the stack must be continuously adjusted to accommodate the removal of sheets from the stack.
Use of air to separate sheets also requires structures that prevent the topmost sheet or sheets of a stack to fly out of position prior to being picked. For this reason, each of the structures disclosed in the '614 and '144 patents includes a flotation suppression member ensuring that the upper sheet is not blown off.
The solutions proposed by the '614 and '144 patents are not well suited at least for larger sheet sizes such as ISO standard sheet sizes B2 (500×707 mm), A2 (420×594 mm), A0 (841×1189 mm), or other larger formats. Such large formats, besides being large (having a surface area of up to 1 m2), may also have a considerable weight of up to 400 gr. per sheet or more. Therefore, the dual fixed position blowers employed by the structures of each of the '614 and '144 patents would be inadequate to provide the necessary air cushion that can keep the upper sheets of the stack floating while still being compact enough and practical for use in stack feeding machines. Further, even if the dual blowers were configured to provide sufficient air flow to lift multiple large sheets from the top of the stack, the increased air flow would likely cause multiple sheets to concentrate below the respective floatation suppression members, which would likely impair sheet separation and cause multi-picks as the concentrated sheets would cling together.
In general, sheet aeration by means of several fixed blowers is known to cause uneven sheet separation insofar as while one part of a sheet closest to the blower may be blown upwards by the blower, other parts of the same sheet disposed away from the blower may be blown downwards by portions of the air stream passing over the sheet, which at least inhibits the creation of a consistent air cushion between the sheets.
An additional example of a previously proposed solution can be seen in DE 102007022700 A1, which in one embodiment describes a blower having a motor-driven impeller wheel that provides an air stream for aerating the side of a stack of sheets in various feeding systems. The blower is positioned at the side of the substrate stack such that the air stream is blown towards to the upper portion of the stack. The blower is connected to a feeding arm that continuously adjusts the position of the blower to be adjacent the top of the stack as sheets are removed.
Therefore, although some previously proposed solutions include sheet separation by aeration, none is capable of automatically adjusting the position of the air stream supply for compensation of the wavelike movement of the inflated sheets, which can cause multi-picks or other stack feeding issues.